Plural beam steering system

ABSTRACT

A system for forming and steering beams of radiation at a plurality of frequencies radiated into a medium capable of producing a nonlinear reaction between these beams resulting in a radiant energy signal having a resultant frequency equal to an arithmetic combination of the radiated frequencies. The beam forming is accomplished by an array of radiating elements arranged preferably in a random fashion to produce a directivity pattern having a main lobe while minimizing the magnitudes of side lobes. The steering is accomplished by variable delay lines coupled between a source of signals at the radiated frequencies and the array of radiating elements providing for individual delays to each of these radiating elements so that each of the beams can be steered with individually controllable steering angles. The delays are varied in accordance with command signals from a beam steering computer to direct the main lobes of the radiation patterns through a common region of the medium as the beams are scanned, this resulting in a scanned beam at the resultant frequency. The signal resulting from the nonlinear reaction may be correlated with a replica thereof, the replica being generated in conjunction with the two radiated frequencies.

nited States Patent Walsh PLURAL BEAM STEERING SYSTEM [75] Inventor:George M. Walsh, Middletown, RI.

[73] Assignee: Raytheon Company, Lexington,

Mass.

[22] Filed: Jan. 15, 1973 [21] Appl. No.: 323,602

[52] U.S. Cl 340/3 R [51] Int. Cl. Gls 9/66 [58] Field of Search 340/3R, 3 FM [56] References Cited UNITED STATES PATENTS 3,613,069 /197]Cary, Jr. et al. 340/3 R Primary Examiner-Richard A. Farley Attorney,Agent, or Firm-M. D. Bartlett; J. D. Pannone; D. M. Warren [57] ABSTRACTA system for forming and steering beams of radiation at a plurality offrequencies radiated into a medium 54 +To CORRELATOR l SIG. GEN,

MIX

[11] 3,824,531 July 16, 1974 capable of producing a nonlinear reactionbetween these beams resulting in a radiant energy signal having aresultant frequency equal to an arithmetic combination of the radiatedfrequencies. The beam forming is accomplished by an array of radiatingelements arranged preferably in a random fashion to produce adirectivity pattern having a main lobe while minimizing the magnitudesof side lobes. The steering is accomplished by variable delay linescoupled between a source of signals at the radiated frequencies and thearray of radiating elements providing for individual delays to each ofthese radiating elements so that each of the beams can be steered withindividually controllable steering angles. The delays are varied inaccordance with command signals from a beam steering computer to directthe main lobes of the radiation patterns through a common region of themedium as the beams are scanned, this resulting in a scanned beam at theresultant frequency. The signal resulting from the nonlinear reactionmay be correlated with a replica thereof, the replica being generated inconjunction with the two radiated frequencies.

13 Claims, 6 Drawing Figures Q i 3 3 3. 2 //2 BEAM FORMING SYSTEM \THHWI I /00 72 I H4 60 62 36 TIMING CLOCK I use .l

t //6 TO DISPLAY 65 74 I OSCILLATOR F TOCORRELATOR J'Ll'l "2 V I0? 82L-] 78 7696 /04 56 BEAM STEERING m COMPUTER }T0 SWITCHES I l 1 l l l l I1 1 l J 4? TRANSDUCER SYSTEM PATENIED JUL I 61974 N S A W E m m M A m ww m A H v 2 .A v

I I I I I I I I I I I I I I I I I I I I I I I I LTRANSDUCER SYSTEMBACKGROUND OF THE INVENTION In the past, numerous experiments have beenconducted for examining a parametric interaction of two beams of sonicenergy which are radiated at two different frequencies through anonlinear medium, this interaction providing a beam of sonic energyradiated at other frequencies, each of which is equal to an arithmeticcombination of the first two frequencies. The other frequencies mostcommonly examined are the sum and difference frequencies. The differencefrequency radiation, as has been disclosed in a copending applicationfor United States patent entitled System for Low- Frequency Transmissionof Radiant Energy, Ser. No 111,218, filed Feb. 1, 1971 by William L.Konrad and Mark A. Chramiec, now abandoned, is particularly useful forproviding low frequency transmission in a narrow beam from a relativelysmall size radiator and furthermore is useful for-penetrating materialsuch as the ocean bottom from which higher frequencies tend to bereflected. It is also known that theattenuation of sonic radiations in afluid medium such as water varies with the frequency of the radiationsuch that lower frequencies experience less attenuation than higherfrequencies. At long ranges from a source of sound where both the highfrequencies and the low frequencies are of relatively low intensitiesdue to the attenuation effects of the medium, the intensity of the lowerfrequency radiation may well be stronger than the intensity of thehigher frequency radiations due to the selective attenuation eventhough, initially, the intensities of the higher frequency radiationswere much greater than the intensity of the lower frequency radiationproduced by the parametric interaction of the higher frequencyradiations. As a practical matter, the intensity of the lower frequencyradiation, as reflected off the sand or muck at the bottom of a harbor,is of such low intensity that detection of the low frequency radiationis obtained by correlation techniques in which the reflected signals arecompared with a replica generated synthetically Maximum utilization ofthe difference frequency radiation requires a capability for steering abeam of this radiation for purposes such as scanning the bottom of aharbor, as well as stabilizing the'beam during the rocking of a boatcarrying equipment for generating the beam. A problem arises in that,since the beam of radi ation at the difference frequency arises from thenonlinear interaction of two beams of radiation at higher frequencies,each of the higher frequency beams must be steered in such a manner thatthe resultant difference frequency beam can be formed and be steered ina desired direction. It is also apparent that a radiating transducer orprojector of sonic energy does not produce a single lobed beam but,rather, produces radiation having a directivity pattern characterized bya main lobe plus a multiplicity of side lobes whose relative amplitudesdepend on factors such as the size of the projector and, if theprojector consists of an array of radiating elements, upon the spacingof these elements. It is readily apparent that in the steering of thetwo beams of higher frequency radiation, each of which is characterizedby a multiple lobed directivity pattern, that care is required to insurethat the side lobes of the respective directivity patterns are sooriented with respect to the projector that there is no substantialoverlapping of the side lobes as might result in the parametricinteraction of the side lobes to produce a multiplicity of differentlyoriented beams of radiation at the difference frequency. An additionalproblem must also be considered, namely, that in any sonar systemutilizing a beam of radiation at the difference frequency, it is 1 mostprobable that some form of signal modulation will be utilized,particularly if correlation techniques are to be employed in thereception of the difference frequency radiation; such modulation mustnecessarily be present on at least one of the high frequency radiationbeams, this presenting the requirement for preserving the temporalrelationships of the modulation on one high frequency beam relative tothe other high frequency beam as these beams are steered about aprojector which may well have a length equal to many wavelengths of thehigh frequency radiations.

SUMMARY OF THE INVENTION The aforementioned problems are overcome andother advantages of beam steering are provided by a system, inaccordance with the invention, which comprises an array of transducerelements which radiate radiant energy at a first frequency and at asecond frequency, the radiating elements being positioned for formingbeams of the radiant energy and directing these beams into a nonlinearmedium such as sea water, to provide a parametric interaction betweenthese beams of energy. This interaction, which is associated with thewave propagation characteristic often referred to as finite amplitude,produces a resultant beam of energy' emanating from a region of themedium which is illuminated simultaneously by the beams of radiation atthe first and at the second frequencies, the resultant radiation havingfrequencies which are equal to arithmetic combinations of the firstand-the second frequency. Of particular interest herein is the resultantbeam having a frequency equal to the difference of the first and thesecond frequencies. The invention further comprises means for generatingsignals at the first and the second frequencies having a desiredmodulation, and means for coupling and selectively delaying each ofthese signals to each of the transducer elements. In one embodiment ofthe invention, the delayed signals at the first frequencyare coupled tohalf of the transducer elements while the delayed signals at the secondfrequency are coupled to the remaining half of the transducer elements,the transducer elements operating at the first frequency beinginterleaved with the transducer elements operating at the secondfrequency so that there is a common phase center for the beams ofradiation produced by the sets of transducer elements operating at thefirst and the second frequencies. A receiving system is also disclosedin which provisions are made for generating a replica of the differencefrequency signal for correlation with a signal received from the mediumat the difference frequency. In an alternative embodiment of theinvention, the delayed signals at the first frequency are summedtogether with the delayed signals at the second frequency and applied tothe radiating elements so that each radiating element transmits both asignal at the first frequency and a signal at the second frequency. Witheither embodiment of the invention, the temporal relationship isretained between the modulation of the signal at the first frequency andthe signal at the second frequency at all points along the array oftransducer elements by virtue of the variable delays, these delays beingprovided by a computer in accordance with interferometric principles.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned aspects and otherfeatures of the invention are explained in the following descriptiontaken in connection with the accompanying drawings wherein:

FIG. 1 is a pictorial view ofa boat carrying a scanning sonar system ofthe invention for scanning the ocean bottom;

FIG. 2 is a block diagram of the scanning sonar system in FIG. 1;

FIG. 3 shows a diagram of one embodiment of an transducer systemcomprising an array of radiating elements for use in the system of FIG.2;

FIG. 4 is a diagram of an alternative embodiment of the transducersystem of FIG. 3;

FIGS. 5 and 6 show respectively the directivity patterns of radiantenergy directed straight away from a projector and at an angle relativeto the projector, a pair of directivity patterns being shown for radiantenergy at a first and at a second frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a system comprising an transducer system 22 positioned at thebottom of a ship 24 for forming a beam 26 of radiant acoustic energy andfor directing the beam 26 towards an object such as driftwood 28submerged in the ocean 30 and towards an object such as a pipe 32 buriedin the sand 34 beneath the ocean 30. The system 20 further comprises abeam forming system 36, a receiving system 38 and a hydrophone 40, thetransducer system 22 being coupled to the beam forming system 36 viaelectrical conductors indicated by lines 42, individual ones of theselines 42 being labeled A A, and B,B,,, as will be more fully explainedin FIG. 2. Timing signals are provided by the beam forming system 36along line 44 to the receiving system 38, and signals from thehydrophone 40 are transmitted along line 46 to the receiving system 38.

As will be disclosed subsequently, the transducer system 22 comprises aprojector array 48 which forms two coincident beams of radiant sonicenergy, the frequencies of these radiations differing slightly. Theamplitudes of these radiations are sufficiently high to generate theaforesaid beam 26 which is at a frequency equal to the differencebetween the two frequencies of the radiations emanating from theprojector array 48, the beam 26 arising through a nonlinear interaction,involving the finite amplitude effect, of the two beams emanating fromthe projector array 48 with the waters of the ocean 30. The widths ofthe beams of energy radiating from the projector array 48 differslightly because of their differing frequencies but are approximatelyequal to the width of the low frequency beam 26. The low frequencyradiations are indicated by waves 50 and 52 which are respectivelyincident upon and reflected by the driftwood 28 and the pipe 32. Thebeam 26 is made to scan the sand 34 at the bottom of the ocean 30 in anovel manner, to be described hereinafter, by means of the beam formingsystem 36 and the transducer system 22 so that data relative tosubmerged objects in the ocean 30 are communicated via the waves 52 tothe hydrophone 40.

Referring now to FIG. 2, there is presented a block diagram of thesystem 20 which shows the transducer system 22, the hydrophone 40, thebeam forming system 36 and the receiving system 38 previously seen inFIG. 1. The beam forming system 36 provides a signal modulation suitablefor sonar operations, two high frequency signals suitable fortransmission from the projector array 48, and means for coupling thesignals to each element of the projector array 48 to form the beam 26.The beam forming system 36 comprises a signal generator 54, anoscillator 56, a mixer 58, a timing unit 60, a clock 62, two clippers 64and 66, two AND gates 68 and 70, two shift registers 72 and 74, and acomputer 76 responsive to clock pulse signals on line 78 from the clock62 and ship orientation signals on line 80 from the ships gyrocompassindicated as gyro 82 in the figure. The beam forming system 36 alsocomprises a set of switches 84 which are coupled to the transducersystem 22 by filters 86, and switches 88 which are coupled to thetransducer system 22 via filters 90. Each of the switches 84 and 88 aredigital multiplexing switches which, in response to a multibit commandsignal from the computer 76, couple selected outputs from respectivelythe shift registers 74 and 72 via the filters 86 and 90 to thetransducer system 22. The outputs from the shift registers 72 and 74 areseen coupled to the switches 84 and 88 via cables 92 and 94. Each of thecables carrying the multibit command signals from the computer 76 to theswitches 84 and 88 comprise a set of parallel lines which are shown inthe figure by a heavy line and identified by the numeral 96, it beingunderstood that each of the cables 96 are usually carrying differentcommand signals to respective ones of the switches 84 and 88.

The signal generator 54 may be of any well-known form for providing asignal modulation suitable for sonar operations, and is shown by way ofexample as a swept frequency oscillator for producing a frequencymodulation. A graphical representation of the signal is shown in theblock representing the signal generator 54. The signal generator 54provides a pulsed sinusoid in which the frequency of the sinusoid isseen to vary during each pulse with a pattern that repeats from pulse topulse. The frequency of the signal is represented by the symbol F thissymbol also serving to identify the line coupling thissignal from thesignal generator 54 to the mixer 58.

The oscillator 56 provides a continuous sinusoidal wave signal to themixer 58 and the clipper 66, this signal being identified by the symbolF and having a frequency P, which is very much greater than thefrequency F The mixer 58 combines the two signals having the frequenciesF and F to provide an output signal on line 98 having the frequency F +Fit being understood that the mixer 58 is of conventional design andincludes a suitable band-pass filter for insuring that only the signalhaving the frequency F +F is coupled to the clipper 64.

The clipper 64 converts the sinusoidal signal appearing on line 98 to asignal having a substantially square waveform on line 100, the squarewaveform having a repetition frequency equal to F F Similarly, theclipper 66 converts the signal on line 102 to a square wave signal online 104. The signals on lines 100 and l 104 are applied respectively toAND gates 68 and 70.

The AND gates 68 and 70 are utilized as sampling circuits for providinga succession of samples for each period of the square wave on line 100and the square wave on line 104. The clock 62 provides clock pulses line104. Since the repetition frequency of the clock pulses on line 106 isvery much greater than that of either the square wave on line 100 or thesquare wave on line 104, for example, approximately 512 clock pulses maybe provided for each period of the square wave on line 100, the sequenceof pulses appearing on line 108 has the form shown in the graph 112 inwhich a sequence of 256 pulses appears over an interval of time equal toone-half the period of the squarewave on line 100, this being followedby an interval of time equal to one-half the period of that squarewavein which no pulses are seen on line 108, thereafter this patternrepeating itself. Clock pulses from the clock 62 are also sent to thetiming unit 60 which comprises suitable countdown circuitry to providesynchronizing pulses on lines 114, 115 and 116 to synchronize theoperation of the signal generator 54 with the sampling by the AND gates68 and 70, the operation of the computer 76, and the operation of adisplay 118 and a correlator 120 which will be described subsequently.

The shift registers 72 and 74 are clocked by the clock pulses on line106 to admit successive pulses in the train of pulses appearing on thelines 108 and 110, respectively. Since the repetition frequencies of thepulses on the lines 100 and 104 are unequal, the repetition frequenciesof the pulses appearing on the lines 108 and 110 are-unequal. It isfurthermore noted that the frequency of the pulses on line 108 varies inaccordance with the'modulation'provided by the signal generator 54, andthat these pulses disappear completely in the intervals between thepulses of the F sinusoid appearing at the output of the signal generator54. Due to the lack of synchronism between the clock pulses on line 106and the square wave on line 100, the number of pulses appearing on line108 for each half cycle of the square wave varies slightly from periodto period of this square wave. Similar comments apply to therelationshipbetween the pulses on line 110 and the pulses on line 104. The pulses online 108 advance through the shift register 72 and are dropped when theyreach the end of the shift register 72; similarly, the pulses on line110 advance through the shift register 74 and are dropped when the reachthe end thereof.

It is apparent that the waveform appearing at any one cell of the shiftregister 72 is identical to the waveform appearing on line 108 exceptthat itis delayed in time, different delays being provided by each cellof the shift register 72. In a similar way, delayed replicas of thepulse train on line 1 appear at successive cells of the shift register74. p

Referring momentarilyto FIG. 3, there is seen a diagrammatic view of oneembodiment of the transducer system 22 in which the projector array 48is seen comprising transducer elements 122. The transducer system 22also comprises a set of power amplifiers 124 of It is noted that thetransducer elements 122 labeled A,A,, are interleaved in a random manneramong the transducer elements 122 labeled B B,,, as describedhereinafter.

Referring now to both FIGS. 2 and 3, it is seen that the outputs of theswitches 84 are coupled via filters 86 along respective ones of thelines 42 labeled A,A,, through respective ones of the power amplifiers124 to the transducer elements 122 labeled respectively A,A,,.Similarly, the outputs of the switches 88 are coupledvia filters 90 viarespective ones of the lines 42 labeled B B,, through respective ones ofthe power amplifiers 124 to the transducer elements 122 labeledrespectively B B,,. In response to signals on respective ones of thelines 96, each of the switches 84 couple replicas of the signal onlineal 10 to the respective filters 86, the amount of delay in thereplica of the signal on line 110 being determined by the particularcell of the shift register 74 that has been selected by the switch 84.

Similarly, the switches 88 select delayed replicas of the signal on line108 and apply them to the filters 90. The filter 86 has a band-passcharacteristic suitable for filtering out frequencies associated withthe sampling frequency or, equivalently, the repetition frequency of theclock pulses on line 106. For example, each filter 86 may be a band-passfilter centered about the frequency F with an upper frequency cutoffwhich is well below a harmonic of the square wave signal on line 104 andalso well below the frequency of the clock pulses on line 106. In thisway, each of the signals appearing on the lines 42 labeled A,A,, aresinusoids having the frequency F but are delayed from the signalappearing on line 110. In a similar manner, the filters are providedwith a band-pass characteristic which passes the frequency F, F butexcludes frequencies of a harmonic of the square wave signal appearingon line and excludes the repetition frequency of the clock pulses online 106. Thus, the signals appearing on the lines 42 labeled Br-B aresinusoids having the frequency F, F are delayed from the signalappearing on line 108. Thus, the transducer elements 122 labeled A A,,are energized with a sinusoid of frequency F while the transducerelements 122 labeled B -B are energized with a sinusoid having afrequency of F, F If desired, the filters 86 and 90 may be eliminated inthose cases where it is desired to use a narrow band pass filtercharacteristic of the transducer elements 122 for filtering out thehigher frequency components of the signal appearing in the outputs ofthe switches 84 and 88. For example, transducer elements of a wellknownpiezoelectric characteristic such as transducer elements of bariumtitanate have a narrow band filter characteristic which may be utilizedin lieu of the filters 86 and 90. However, the filters 86 and 90 arepreferred in that they minimize the chance of any intermodulationdistortion in the set of power amplifiers 124.

The transducer elements 122 labeled A A, may be separated by a spacingof one-half wavelength at the frequency F and are interleaved among thetransducer elements 122 labeled B,B,, which are similarly spaced part bya spacing of one-half wavelength at the frequency F, F This interleavingis done in a random fashion to minimize the amplitudes of side lobesappearing in the directivity patterns of radiations at the frequency Fand F, F In addition, the half wavelength spacing also reduces themagnitude of these side lobes. As is known from antenna theory, thesespacings may be made still smaller to further reduce the amplitude ofthe side lobes. However, it is interesting to note that because of theutilization of the finite amplitude effects, the interelement spacingbetween the transducer elements 122 labeled A A,, and the interelementspacing between the transducer elements labeled B,B,, may be increasedup to a full wavelength and even beyond producing multiple lobeddirectivity patterns in which the intensities of the side lobes arerelatively high compared to the main lobe as will be describedhereinafter with reference to FIGS. 5 and 6. The directivity patternshaving side lobes of minimal amplitudes are preferred since they placemore power in the main lobe where it is more efficiently utilized.

The sonic radiation emanating from the projector array 48 at thefrequency F radiates outwardly in a direction normal to the face of theprojector array 48 when the switches 84 have selected equal delays foreach of the signals on the lines 42 labeled A,A,,. When these delayshave been selected such that the signals emanating from the transducerelements 122 near one end of the projector array 48 have a greater delaythan the signals emanating from the opposite end of the projector array,it being pressumed that there is a uniform delay taper across the faceof the array 48 and that the amount of delay experienced by the signalof any one transducer element 122 is proportional to the distance ofthattransducer element from the end of the array experiencing the minimaldelay, then the radiation emanating at the frequency F is directed at anangle away from the normal to the array base. By suitably selecting thedelay for each of the transducer elements 122 labeled A,A,,, the beam ofacoustic energy radiated at the frequency F may be steered about twoaxes, namely, the roll axis and pitch axis of the ship 24 of FIG. 1.Similar comments apply to the sonic energy radiated at the frequency F,F

As shown in FIG. 2, the receiving system 38 comprises a preamplifier126, the correlator 120 and display 118. Acoustic energy radiated fromthe projector array 48 and reflected off the driftwood 28 and pipe 32 isreceived by the hydrophone 40, which may be of conventional design, andamplified by the preamplifier 126. The computer 76 provides beamsteering commands simultaneously for both the beams of acoustic energyradiated at the frequencies F and F, F so that the main lobes of theirrespective directivity patterns are directed in the same direction. Theacoustic energies at these two frequencies ensonify the water of theocean 30 with sufficient intensity to induce the nonlinear finiteamplitude reaction which results in the generation of acoustic energiesat a number of frequencies each of which is equal to an arithmeticcombination of the frequencies F and F, F The sonic radiation producedat the difference of these two frequencies, namely, F,, is particularlyuseful in that it is attenuated far more slowly than the higherfrequency radiations and grows in relative amplitude with increasingdistance from the projector array 48. Of particular interest is the factthat this low frequency radiation can penetrate the sand 34 and detectsubmerged objects such as the pipe 32 more readily than the higherfrequency radiations which reflect off the bottom surface of the ocean30. The hydrophone 40 is designed with a band-pass characteristicsuitable for receiving the sonic energy at the frequency F,, and thepreamplifier 126 has a similar band-pass characteristic for amplifyingthe signals. The display 118 may comprise a cathode-ray tube, and theoutput signal of the preamplifier 126 appearing on line 128 may betransmitted directly (not shown in the figures) to the display 118 to bevisualized. Since the frequency F, is typically in the audio range, thedisplay 118 may comprise a set of earphones (not shown) to permitlistening directly to the signals reflected from the driftwood 28 andthe pipe 32, the frequency modulation of the signal aiding inidentification thereof. However, at depths normally encountered inharbors and at greater depths, the signals received at the differencefrequency F, may well be excessively small compared to background noise,this precluding direct displaying of these signals on the display 118.In these situations, the correlator is utilized and a replica isprovided on line 130 from the signal generator 54 for comparison withthe signal on line 128. Typically, digital correlators are utilized inwhich case timing pulses on line 116 are provided for operating thecorrelator 120. The output of the correlator 120 is then applied to thedisplay 118.

Referring now to FIG. 4, there is shown an alternative embodiment of thetransducer system 22 of FIG. 2, identified by the legend 22A in FIG. 4.Amplifiers 132 are provided for coupling the signals on the lines 42,seen also in FIG. 2, to individual transducer elements 134 whichcollectively compose a projector array 136. Each of the amplifier 132sums together the signals on the lines 42 such that the signal on theline A, is added to the signal on the line B,, the signal on line A isadded to the signal B and similarly with the remaining lines through A,,and B,,. In this way each of the transducer elements 134 radiate sonicenergy at both of the frequencies F and F, F The computer 76 provides aset of beam steering commands different from that provided for theprojector array 48 of FIG. 3 since the sonic energy is radiated from adifferent set of locations in the case of the projector array 136.

Referring now to FIGS. 5 and 6, thereis seen the directivity patterns ofthe sonic energy radiated from the projector array 48 of FIG. 2 in whichthe main lobe is radiated in a direction normal to the array in FIG. 5and at an angle off the axis, or normal, of the projector array 48 inFIG. 6. The directivity pattern formed by the solid line identified bythe number 138A in FIG. 5 and 138B in FIG. 6 represents the radiationsat the frequency F2, while the directivity patterns formed by the dashedlines identified by the legends 140A in FIG. 5 and 140B in FIG. 6represent the sonic energy radiated at the frequency F, F Threedirectivity patterns have been drawn for the situation wherein theinterelement spacing is greater than a wavelength to accentuate the sidelobes. Of particular interest here is the fact that while the main lobesoverlap in both FIGS. 5 and 6, the side lobes do not overlap, thedirectivity patterns differing because of the differenting wavelengthsof the two radiations. The finite amplitude effect is significantlyreduced for side lobes because their intensity is lower than that of themain lobe. Furthermore, due to the lack of spatial coincidence of theside lobes at one frequency versus the side l obes at the otherfrequency, there is a still further reduction in the finite amplitudeeffect produced by the interaction of acoustic energies radiated by theside lobes. Accordingly, a directivity pattern (not shown) drawn for thedifference frequency F would show a preponderance of the main lobe overthe side lobes even though the directivity patterns of the higherfrequency acoustic energies which induce the difference frequencyradiation have substantial side lobes. For this reason, a highlydirective beam of the difference frequency radiation can be readilysteered by the system 20 of FIGS. 1 and 2 while retaining itsdirectivity at steering angles without introducing the familiar gratinglobe pattern associated with phased arrays in both sonar and radarsystems. It is interesting to note, that this discussion of the finiteamplitude effect is equally applicable to the nonlinear effects producedby radiations in media other than water, be it a fluid medium such asair or a solid medium.

It is understood that the above-described embodiments of the inventionare illustrative only and that modifications thereof will occur to thoseskilled in the art. Accordingly, it is desired that this invention isnot to be limited to the embodiments disclosed herein but it is to belimited only as defined by the appended claims.

What is claimed is:

l. A scanning sonar system comprising:

means for radiating sonic energy in a first and in a second sonicradiation pattern respectively at a first and a second frequency in amedium capable of producing nonlinear acoustic effects, each of saidradiation patterns having a main lobe and side.

lobes, each of said main lobes being directed through a common region ofsaid medium, the side lobes of said first radiation pattern ensonifyingregions of said medium separate from regions of said medium ensonifiedby side lobes of said second radiation pattern; means for altering thedirections of each of said main lobes and said side lobes relative tosaid radiating means, each of said altered main lobes being directedthrough a common region of said medium, the altered side lobes of saidfirst radiation pattern ensonifying regions of said medium separate fromregions of said medium ensonified by altered side lobes of said secondradiation pattern; and means coupled to said radiating means forgenerating said sonic energy at a sufficiently high intensity level forproviding a nonlinear reaction in the common region of said mediumensonitied by said main lobes wherein radiant acoustic energy isprovided at a third frequency equal to an arithmetic combination of saidfirst and said second frequencies. 2. A system according toclaim 1wherein said altering means comprises means for delaying a signalradiated from a portion of said radiating means relative to a signalradiated from another portion of said radiating means.

- 3. A system according to claim 2 wherein said delaying means comprisesa multiply tapped delay medium and a plurality of switchesinterconnecting respective ones of said taps with respective portions ofsaid radiating means.

4. A system according to claim 3 wherein said delay medium comprises apair of shift registers. i

5. The system according to claim 4 comprising means for generating agenerally square shaped waveform at said first frequency and a generallysquare shaped waveform at said second frequency, and means for samplingsaid waveform at said first frequency and said waveform at said secondfrequency at a sampling rate higher than said first frequency and higherthan said second frequency, said sampling means being coupled to saidshift registers.

6. A system according to claim 5 including oscillator means forproviding said waveform at said first frequency, said system furthercomprising a signal generator and means for combining an output of saidsignal generator with an output of said oscillator means to provide saidwaveform at said second frequency.

7. In combination:

a plurality of radiating elements positioned for coupling radiant energyinto a medium capable of inducing a nonlinear reaction between waves ofsuch radiant energy propagating through said medium;

first means for energizing a plurality of said radiating elements with asignal at a first frequency;

second means for energizing a plurality of said radiating elements witha signal at a second frequency and an intensity which is sufficientlyhigh to induce said nonlinear reaction in said medium between waves ofthe energies at said first frequency and said second frequency;

said first energizing means including means coupled to respective onesof said radiating elements for steering a wave front of radiation atsaid first frequency; and

said second energizing means including means coupled to respective onesof said radiating elements for steering a wave front of radiation atsaid second frequency in a direction for intercepting a region of saidmedium illuminated by said radiation at said first frequency forproviding said nonlinear reaction in said commonly illuminated region,said nonlinear reaction resulting in a signal radiated at a thirdfrequency different from said first and said second frequency.

8. A combination according to claim 7 wherein said first energizingmeans includes means for providing a sample of said signal at said thirdfrequency, said sample being suitable for a correlation of said samplewith said signal radiated at said third frequency.

9. A combination according to claim 8 wherein said second energizingmeans includes an oscillator for providing a signal at said secondfrequency.

10. A combination according to claim 9 wherein said first energizingmeans includes means for combining said signal of said oscillator withsaid sample to provide a signal at said second frequency.

ll. The combination according to claim 10 wherein said first energizingmeans and said second energizing means includes means for sampling saidsignal of said oscillator and said signal of said combining means.

12. A combination according to claim 11 wherein said steering means ofsaid first energizing means includes a delay medium coupled to thesampling means of said first energizing means, said delay mediumproviding a set of delays of said sampled signal, said steering means ofsaid first energizing means further comprising means for selectivelycoupling delays of said delay medium to said respective ones of saidradiating elements positioned for coupling radiant energy at saidelements.

13. A combination according to claim 7 wherein said plurality ofradiating elements are positioned in an array, said array of radiatingelements including radiating third frequency from said medium forreceiving said radiant energy.

1. A scanning sonar system comprising: means for radiating sonic energyin a first and in a second sonic radiation paTtern respectively at afirst and a second frequency in a medium capable of producing nonlinearacoustic effects, each of said radiation patterns having a main lobe andside lobes, each of said main lobes being directed through a commonregion of said medium, the side lobes of said first radiation patternensonifying regions of said medium separate from regions of said mediumensonified by side lobes of said second radiation pattern; means foraltering the directions of each of said main lobes and said side lobesrelative to said radiating means, each of said altered main lobes beingdirected through a common region of said medium, the altered side lobesof said first radiation pattern ensonifying regions of said mediumseparate from regions of said medium ensonified by altered side lobes ofsaid second radiation pattern; and means coupled to said radiating meansfor generating said sonic energy at a sufficiently high intensity levelfor providing a nonlinear reaction in the common region of said mediumensonified by said main lobes wherein radiant acoustic energy isprovided at a third frequency equal to an arithmetic combination of saidfirst and said second frequencies.
 2. A system according to claim 1wherein said altering means comprises means for delaying a signalradiated from a portion of said radiating means relative to a signalradiated from another portion of said radiating means.
 3. A systemaccording to claim 2 wherein said delaying means comprises a multiplytapped delay medium and a plurality of switches interconnectingrespective ones of said taps with respective portions of said radiatingmeans.
 4. A system according to claim 3 wherein said delay mediumcomprises a pair of shift registers.
 5. The system according to claim 4comprising means for generating a generally square shaped waveform atsaid first frequency and a generally square shaped waveform at saidsecond frequency, and means for sampling said waveform at said firstfrequency and said waveform at said second frequency at a sampling ratehigher than said first frequency and higher than said second frequency,said sampling means being coupled to said shift registers.
 6. A systemaccording to claim 5 including oscillator means for providing saidwaveform at said first frequency, said system further comprising asignal generator and means for combining an output of said signalgenerator with an output of said oscillator means to provide saidwaveform at said second frequency.
 7. In combination: a plurality ofradiating elements positioned for coupling radiant energy into a mediumcapable of inducing a nonlinear reaction between waves of such radiantenergy propagating through said medium; first means for energizing aplurality of said radiating elements with a signal at a first frequency;second means for energizing a plurality of said radiating elements witha signal at a second frequency and an intensity which is sufficientlyhigh to induce said nonlinear reaction in said medium between waves ofthe energies at said first frequency and said second frequency; saidfirst energizing means including means coupled to respective ones ofsaid radiating elements for steering a wave front of radiation at saidfirst frequency; and said second energizing means including meanscoupled to respective ones of said radiating elements for steering awave front of radiation at said second frequency in a direction forintercepting a region of said medium illuminated by said radiation atsaid first frequency for providing said nonlinear reaction in saidcommonly illuminated region, said nonlinear reaction resulting in asignal radiated at a third frequency different from said first and saidsecond frequency.
 8. A combination according to claim 7 wherein saidfirst energizing means includes means for providing a sample of saidsignal at said third frequency, said sample being suitable for acorrelation of said sample with said signal radiated at said thirdfrequency.
 9. A combination according to claim 8 wherein said secondenergizing means includes an oscillator for providing a signal at saidsecond frequency.
 10. A combination according to claim 9 wherein saidfirst energizing means includes means for combining said signal of saidoscillator with said sample to provide a signal at said secondfrequency.
 11. The combination according to claim 10 wherein said firstenergizing means and said second energizing means includes means forsampling said signal of said oscillator and said signal of saidcombining means.
 12. A combination according to claim 11 wherein saidsteering means of said first energizing means includes a delay mediumcoupled to the sampling means of said first energizing means, said delaymedium providing a set of delays of said sampled signal, said steeringmeans of said first energizing means further comprising means forselectively coupling delays of said delay medium to said respective onesof said radiating elements.
 13. A combination according to claim 7wherein said plurality of radiating elements are positioned in an array,said array of radiating elements including radiating elements positionedfor coupling radiant energy at said third frequency from said medium forreceiving said radiant energy.